Fiber- and fabric-reinforced resins have achieved broad usage because of their relatively high strength-to-weight ratio and have been incorporated in articles as diverse as automobiles and tennis racquets. Many polymers have been used as matrix resins, such as polycarbonates, epoxy resin, and polyesters generally.
It is highly desirable that such reinforced resins be able to be worked or molded in the field without loss of strength. One approach to this goal has been to use a low melting polymer which can be cured or cross-linked to a thermostable resin upon being melted, thereby permitting working and shaping concurrent with attainment of maximum strength.
Where cross-linking involves a chemical reaction which liberates a volatile by-product, voids may be produced in the resulting polymer leading to reduction in strength and limitations on shaping. This disadvantage has been overcome using the biphenylene nucleus as a cross-linking entity. Difunctionalized biphenylene monomers have been incorporated into polyquinoxalines (Garapon and Stille, Macromolecules, 10, 627 (1977); Recca and Stille, ibid., 10, 1344 (1977) and 11, 479 (1978)), polyamides (Swedo and Marvel, J. Polymer Sci., Polymer Chem. Ed., 16, 2711 (1978)) and polyaromatic ether-ketones and ether-ketone sulfones (idem., ibid., 15, 683 (1977) and 17, 2815 (1979)). In U.S. Pat. No. 4,197,393 the patentees incorporated the biphenylene nucleus into the polymer chain itself and upon subsequent heating cross-linking occurred via the formed biphenylene diradical to afford a thermostable resin. U.S. Pat. No. 4,269,953 presents the variant wherein the biphenylene nucleus is incorporated as an additive which causes cross-linking of a thermoplastic polymer upon heating.
The solutions embodied in the cited art suffer from the disadvantage that the source of the biphenylene entity in each case is inordinately expensive. For example, the patentees of U.S. Pat. No. 4,197,393 require a biphenylene dicarboxylic acid halide, available only via a multi-step synthesis from biphenylene. The patentee of U.S. Pat. No. 4,269,953 requires additives of the structure ##STR1## whose availability also is limited by the necessity of its multi-step synthesis from the parent biphenylene.
It is thus apparent that there is a need for biphenylene itself to be the source of the biphenylene nucleus as a cross-linking entity. It is not feasible to use biphenylene merely as an additive because its relatively high vapor pressure would lead to its excessive loss by sublimation upon melting of the polymer to which it had been added. Similarly, it is to be expected that using biphenylene in an approach analogous to that described in U.S. Pat. No. 4,197,393 would be unsuccessful based on the report (Blatchly et al., J. Chem. Soc., 1962, 5090) that biphenylene itself undergoes Friedel-Crafts mono-benzoylation in poor yield (59%) and in light of the requirement that reactions in a step-growth polymerization need to be nearly quantitative. The discovery which makes this invention possible is that biphenylene can be used as an acylating substrate in a step-growth polymerization. Quite surprisingly, it has been observed that biphenylene can be reacted in high yield in the aforesaid polymerization to afford polymers of reasonable chain length.
An important advantage of the invention described herein is the use of the substantially less expensive unsubstituted biphenylene in place of its derivatives.
Another advantage is that the polymers of this invention have biphenylene flanked by aromatic structures originally bearing the dicarboxylic acid halide, in contrast to prior art polymers where the biphenylene nucleus is flanked by ether or ether-sulfone structures.
Accordingly, the purpose of the invention described herein is to provide polymers which undergo cross-linking upon being melted, the cross-linking occurring via biphenylene diradicals arising through unsubstituted biphenylene being incorporated into the polymer chain, and a method of making such polymers.